One of my L. karasmontana got attacked from underneath and eaten away. I only saw a tiny (2 mm) insect when I uprooted the plant. I doubt one insect alone could have caused so much damage. Nearby Lithops look fine. The first picture was taken in Feb 17 when I bought the plant. The last picture shows how transparent the upper skin of a Lithops is (the orange patch). Has anybody experienced a similar problem?

My only experience with this kind of damage is with caterpillars, though never eaten in such a way. Some of them have nocturnal activity, hiding during the day. I live in the other side of the world, so who knows what creatures do you have in Australia I would have a look at night because they never come alone. Good luck!